1. Field of the Invention
The present invention generally relates to a communication system, and more particularly to a method and apparatus for performing a time windowing process to transmit a signal in a Broadband Wireless Access (BWA) communication system based on an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
Conventionally, Orthogonal Frequency Division Multiple Access (OFDMA) is a scheme for efficiently dividing and allocating frequency resources for users in a multi-user environment on the basis of an Orthogonal Frequency Division Multiplexing (OFDM) scheme. The OFDM scheme divides a data stream with a high transmission rate into multiple data streams with a low transmission rate and simultaneously transmits the multiple data streams in parallel using multiple subcarriers. This OFDM scheme has a high data rate and high spectral efficiency, and is robust to a frequency fading channel.
The OFDM scheme inserts a guard interval longer than the channel delay spread between OFDM symbols (“symbols”) to prevent the orthogonality between subcarriers from being destroyed, thereby removing Inter-Symbol Interference (ISI). To ensure the continuity of a total symbol interval including the guard interval, a Cyclic Prefix (CP) is inserted into the guard interval. That is, when part of a symbol is copied and inserted into the guard interval as the CP and is added to a start part of the symbol, the symbol is cyclically extended, such that Inter-Carrier Interference (ICI) can be avoided.
The OFDM scheme implements a parallel subcarrier transmission with an Inverse Fast Fourier Transform (IFFT) at a transmitting side and a Fast Fourier Transform (FFT) at a receiving side. Subcarriers of an OFDM signal are constructed with a sinc function and overlap while maintaining the orthogonality between the subcarriers. Because the OFDM signal is not a band limited signal due to characteristics of the sinc function, it may interfere with an adjacent band.
To reduce the interference to the adjacent band, a method does not transmit data on all subcarriers in a frequency band, but never transmits a signal on some subcarriers at both ends of an associated band. However, because the side lobe of the sinc function itself is relatively large, the number of subcarriers on which no data is transmitted should significantly increase to remove the interference to the adjacent band. However, in this case, the spectral efficiency is significantly degraded.
Therefore, time windowing is mainly used to reduce interference with an adjacent band while maintaining the spectral efficiency. When the time windowing is used, the side lobe can be effectively reduced. Among many windows used in the windowing scheme, a raised cosine window is widely used.
Raised cosine windowing schemes using the raised cosine window are raised cosine windowing of a 1-symbol interval (a “1-symbol interval windowing scheme”) and raised cosine windowing of a 3-symbol interval (a “3-symbol interval windowing scheme”). The 1-symbol interval windowing scheme will be described with reference to FIG. 1.
Referring to FIG. 1, Ts is a 1-symbol interval as a symbol period, Tg is a guard interval, and Tb is an effective symbol interval. As illustrated in FIG. 1, one symbol is constructed with the guard interval Tg and the effective symbol interval Tb subsequent thereto. As described above, a rear part to be used for the guard interval Tg within the effective symbol interval Tb is copied and inserted as a CP.
Assuming that a time domain OFDM signal to be transmitted is x(n), a transmission signal s(n) windowed in the 1-symbol interval windowing scheme is defined as shown in Equation (1). A time windowing coefficient w(n) is defined as shown in Equation (2).
                                          s            ⁡                          (              n              )                                =                                    w              ⁡                              (                n                )                                      ×                          x              ⁡                              (                n                )                                                    ,                              for            ⁢                                                                      ⁢                                                                    ⁢            0                    ≤          n          ≤                                    N              s                        -            1                                              Equation        ⁢                                  ⁢                  (          1          )                                                  w          ⁡                      (            n            )                          =                  (                                                                                          0.5                    ×                                          (                                              1                        +                                                  cos                          ⁡                                                      (                                                          π                              ×                                                              (                                                                  1                                  +                                                                      n                                    m                                                                                                  )                                                                                      )                                                                                              )                                                        ,                                                                              0                  ≤                  n                  ≤                  m                                                                                                      1                  ,                                                                              m                  ≤                  n                  <                                                            N                      s                                        -                    m                                                                                                                                            0.5                    ×                                          (                                              1                        +                                                  cos                          ⁡                                                      (                                                          π                              ×                                                                                                n                                  -                                                                      (                                                                                                                  (                                                                                                                              N                                            s                                                                                    -                                          1                                                                                )                                                                            -                                      m                                                                        )                                                                                                  m                                                                                      )                                                                                              )                                                        ,                                                                                                                        N                      s                                        -                    m                                    ≤                  n                  ≤                                                            N                      s                                        -                    1                                                                                                          Equation        ⁢                                  ⁢                  (          2          )                    
In Equations (1) and (2), Ns is the number of time samples with respect to the symbol period Ts, and m is a window size.
As shown in Equations (1) and (2), it can be seen that a transmitter of the OFDM system performs a windowing process of the 1-symbol interval by multiplying an interval from a start point of the symbol period Ts to a first window size m (“a first time”) by
  0.5  ×      (          1      +              cos        ⁡                  (                      π            ×                          (                              1                +                                  n                  m                                            )                                )                      )  with respect to a signal of the symbol period Ts in FIG. 1, multiplying an interval up to (Ns−m) (“a second time”) after the first time by 1, and multiplying an interval up to an end of the symbol period after the second time by
  0.5  ×            (              1        +                  cos          ⁡                      (                          π              ×                                                n                  -                                      (                                                                  (                                                                              N                            s                                                    -                          1                                                )                                            -                      m                                        )                                                  m                                      )                              )        .  Because the interval up to the second time after the first time is multiplied by 1, the resulting signal is equal to an original signal. The interval from the start of the symbol period Ts to the first time and the interval from the second time to the end of the second time are windowing intervals in which the original signal is actually distorted by the windowing.
The 3-symbol interval windowing scheme will be described with reference to FIG. 2.
Referring to FIG. 2, Ts is a 1-symbol interval as a symbol period, Tg is a guard interval, and Tb is an effective symbol interval. As illustrated in FIG. 2, the 3-symbol interval windowing scheme is a windowing scheme for overlapping a prefix Tprefix and postfix Tpostfix of the current symbol with a signal of the previous symbol and a signal of the next symbol.
Assuming that a time domain OFDM signal to be transmitted is x(n), a transmission signal s(n) windowed in the 3-symbol interval windowing scheme is defined as shown in Equation (3). A time windowing coefficient w(n) is defined as shown in Equation (4).
                                          s            ⁡                          (              n              )                                =                                    w              ⁡                              (                n                )                                      ×                                          ∑                                                      k                    =                                                                  -                                                  N                          used                                                                    /                      2                                                                            k                    ≠                    0                                                                                        N                    used                                    /                  2                                            ⁢                                                b                  k                                ⁢                                  exp                  ⁡                                      (                                                                  (                                                  j2π                          ⁢                                                                                                          ⁢                          k                          ⁢                                                                                                          ⁢                          Δ                          ⁢                                                                                                          ⁢                          f                                                )                                            ⁢                                              (                                                  n                          -                                                      N                            s                                                                          )                                                              )                                                                                      ,                                  ⁢                                  ⁢                                            for                        ⁢                                                  -            m                    ≤          n          ≤                                    N              s                        +            m                                              Equation        ⁢                                  ⁢                  (          3          )                                                  w          ⁡                      (            n            )                          =                  (                                                                                          0.5                    ×                                          (                                              1                        +                                                  cos                          ⁡                                                      (                                                          π                              ×                                                              (                                                                  1                                  +                                                                                                            n                                      +                                      m                                                                                                              2                                      ⁢                                      m                                                                                                                                      )                                                                                      )                                                                                              )                                                        ,                                                                                                  -                    m                                    ≤                  n                  <                  m                                                                                                      1                  ,                                                                              m                  ≤                  n                  ≤                                                            N                      s                                        -                    m                                                                                                                                            0.5                    ×                                          (                                              1                        +                                                  cos                          ⁡                                                      (                                                          π                              ×                                                                                                n                                  -                                                                      (                                                                                                                  N                                        s                                                                            -                                      m                                                                        )                                                                                                                                    2                                  ⁢                                  m                                                                                                                      )                                                                                              )                                                        ,                                                                                                                        N                      s                                        -                    m                                    <                  n                  ≤                                                            N                      s                                        -                    m                                                                                                          Equation        ⁢                                  ⁢                  (          4          )                    
In Equations (3) and (4), Ns is the number of time samples with respect to the symbol period Ts, and m is a window size. bk is a frequency domain signal to be transmitted on a k-th subcarrier, Ng is the number of time samples during the guard interval Tg, and Nused is the number of subcarriers except virtual subcarriers on which no signal is transmitted among all subcarriers mapped to an IFFT size. That is, Nused is the number of subcarriers to which a pilot or data can be allocated among all the subcarriers mapped to the IFFT size.
Because a signal is artificially distorted in the above-described windowing scheme, there is a problem in that Error Vector Magnitude (EVM) of the system is degraded and hardware complexity increases due to windowing. Specifically, a scheme for performing a windowing process during a 1-symbol interval is simply implemented and has a superior effect on sidelobe attenuation, but has a bad effect on EVM performance.
In detail, EVM is defined as shown in Equation (5), and becomes a measure of modulation accuracy in a transmitter. EVM is an important parameter for implementing a transmission system together with a spectrum mask and should always satisfy conditions predefined in the standard of a particular OFDM system.
                    EVM        =                                                            1                N                            ⁢                                                ∑                  i                  N                                ⁢                                  (                                                            Δ                      ⁢                                                                                          ⁢                                              I                        2                                                              +                                          Δ                      ⁢                                                                                          ⁢                                              Q                        2                                                                              )                                                                    S              max              2                                                          Equation        ⁢                                  ⁢                  (          5          )                    
In Equation (5), Smax2 is the maximum magnitude of the outermost constellation point among constellation points, ΔI2 is an error vector of real, i.e., in-phase axes, ΔQ2 is an error vector of imaginary axes, i.e., quadrature phase axes, and N is the number of subcarriers.
FIG. 3 illustrates EVM measurement results according to window size of the 1-symbol interval windowing scheme.
Referring to FIG. 3, it can be seen that EVM degrades as the window size increases when the window size m is 4, 8, 12, 16, 24, and 32 with respect to a transmission signal of a modulation scheme such as Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM). Thus, it is difficult for the 1-symbol interval windowing scheme to satisfy the EVM condition.
On the other hand, the 3-symbol interval windowing scheme has a lower degradation level in the EVM performance in comparison with the 1-symbol interval windowing scheme, but has a problem in implementation. In other words, because the 3-symbol interval windowing scheme should know a signal of the next symbol to transmit the current symbol, a processing delay occurs during a 1-symbol interval. Thus, there is a problem in that control logic and buffer sizes increase. Furthermore, there is another problem in that hardware complexity increases when the 3-symbol interval windowing scheme is used.